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We all know and love the moon. We're so assured that we only have one that we don't even give it a specific name. 

But the moon is not the Earth's only natural satellite. As recently as 1997, we discovered that another body, 3753 Cruithne, is what's called a quasi-orbital satellite of Earth.

Now researchers believe its strange orbit may help scientists better understand how gravity helped planets form.

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A representation of Cruithne's strange orbit around the sun.  Cruithne scuttles around the inner solar system in what's called a 'horseshoe' orbit. This could provide an ideal testing ground for our understanding of how the solar system evolves under gravity

Cruithne doesn't loop around the Earth in a nice ellipse in the same way as the moon, or indeed the artificial satellites we loft into orbit.

Instead, Cruithne scuttles around the inner solar system in what's called a 'horseshoe' orbit.

To help understand why it's called a horseshoe orbit, imagine we're looking down at the solar system, rotating at the same rate as the Earth goes round the sun. 

From our viewpoint, the Earth looks stationary. A body on a simple horseshoe orbit around the Earth moves toward it, then turns round and moves away. 

Once it's moved so far away it's approaching Earth from the other side, it turns around and moves away again. 

Horseshoe orbits are quite common for moons in the solar system. Saturn has a couple of moons in this configuration, for instance.

Cruithne, a 3 mile (5km) asteroid (animations shown above) is shown here. The yellow orbit in the animation shows Cruithne's orbit around Earth

What's unique about Cruithne is how it wobbles and sways along its horseshoe.

If you look at Cruithne's motion in the solar system, it makes a messy ring around Earth's orbit, swinging so wide that it comes into the neighbourhood of both Venus and Mars. 

So what can we learn about the solar system from Cruithne? Quite a lot. 

Like the many other asteroids and comets, it contains forensic evidence about how the planets were assembled.

Its kooky orbit is an ideal testing ground for our understanding of how the solar system evolves under gravity.

It wasn't until the end of the 20th century that we even realised that bodies would enter such weird horseshoe orbits and stay there for such a long time. 

The fact they do shows us that such interactions will have occurred while the solar system was forming.

Earth's 'other moon' Cruithne's strange orbit

Because we think terrestrial planets grow via collisions of bodies of Cruithne-size and above, this is a big new variable.

One day, Cruithne could be a practice site for landing humans on asteroids, and perhaps even mining them for the rare-earth metals our new technologies desperately crave. 

If Cruithne struck the Earth, though, that would be an extinction-level event, similar to what is believed to have occurred at the end of the Cretaceous period. 

Luckily it's not going to hit us anytime soon – its orbit is tilted out of the plane of the solar system, and astrophysicists have shown using simulations that while it can come quite close, it is extremely unlikely to hit us. 

The point where it is predicted to get closest is about 2,750 years away.

If you look at Cruithne's motion in the solar system, it makes a messy ring around Earth's orbit, swinging so wide that it comes into the neighbourhood of both Venus (pictured) and Mars

Cruithne is expected to undergo a rather close encounter with Venus in about 8,000 years, however. 

There's a good chance that that will put paid to our erstwhile spare moon, flinging it out of harm's way, and out of the Terran family.

The story doesn't end there. Like a good foster home, the Earth plays host to many wayward lumps of rock looking for a gravitational well to hang around near. 

Astronomers have actually detected several other quasi-orbital satellites that belong to the Earth, all here for a little while before caroming on to pastures new. 

Cruithne and these other satellites teach us that the solar system isn't eternal – and by extension, neither are we.

This article originally appeared in The Conversation. It was written by Dr Duncan Forgan who is a postdoctoral research fellow at the University of St Andrews' School of Physics and Astronomy.